Discrete vacuum ultra violet reflective interference filter

Abstract

A thin film reflecting interference filter (RIF) is designed to suppress unwanted harmonics thereby improving the monochromaticity of the radiation. An interference layer of material which has a well-defined plasma oscillation is deposited on a substrate and a mismatch layer is formed thereon. This interference layer exploits the interference between wavefronts reflected from the layer-substrate and the vacuum-layer interfaces to suppress higher order harmonics, while allowing good reflectance at the fundamental wavelength. This is achieved by positioning the RIF in the radiation at an angle of incidence which is greater than the critical angle of the desired fundamental wavelength, but less than critical angles of the harmonics to be suppressed. The mismatch layer increases the reflectance of the unwanted harmonics at the vacuum-layer interface, thus allowing more complete destructive interference of the unwanted harmonics. The RIF can be tuned by altering the thickness of the layer, the composition of the layer and/or the angle of incidence of the radiation.

What is claimed is: 
     
       1. A vacuum ultraviolet reflecting interference filter device comprising: a substrate;   an interference layer, deposited on said substrate so as to create a layer-substrate interface and an ambient-layer interface, said interference layer comprising material having a critical wavelength longer than a desired fundamental wavelength such that wavefronts incident on said device at angles less than a wavelength dependent critical angle are reflected from said layer-substrate interface and said ambient-layer interface, resulting in interference between reflected wavefronts, and such that wavefronts incident on said device at angles greater than said wavelength dependent critical angle reflected only from said ambient-layer interface; and   a mismatch layer, having optical constants more different from optical constants of said material than the difference between ambient optical constants and optical constants of said material, formed on top of said interference layer;   wherein said material has a well-defined plasma oscillation.   
     
     
       2. A device as recited in claim 1, said mismatch layer being a naturally oxidized layer of said material. 
     
     
       3. A device as recited in claim 1, wherein said substrate comprises silicon dioxide. 
     
     
       4. A device as recited in claim 1, said material being selected from the group of consisting of silicon, boron, beryllium, aluminum, tin, magnesium, germanium, indium and carbon. 
     
     
       5. A device as recited in claim 3, said material comprises silicon and said interference layer being 600 Å thick. 
     
     
       6. A device as recited in claim 1, wherein said mismatch layer is an order of magnitude thinner than said interference layer. 
     
     
       7. An interference reflector for reflecting one optical wavelength λ 1  and rejecting another optical wavelength λ 2 , said filter comprising: a substrate;   an interference layer having a well-defined plasma oscillation; and   a mismatch layer;   wherein said substrate is optically mismatched to said interference layer, said mismatch layer is optically mismatched to said interference layer, n<k for said mismatch layer at said λ 2 , said plasma oscillation causes the critical wavelength λ c  of said interference layer to be between said λ 1  and said λ 2  such that λ 2  <λ c  <λ 1  ; and   wherein the thicknesses of said interference layer and said mismatch layer are selected to cause the reflectance of said reflector to be about at a reflective minimum at said λ 2 .   
     
     
       8. The filter of claim 7, wherein: said interference layer is silicon, said mismatch layer is SiO 2 , and   the thickness of said interference layer is about 600 Å.   
     
     
       9. The filter of claim 7, wherein said λ 2  is in the vacuum ultraviolet. 
     
     
       10. An interference filter for filtering light having a preselected light frequency, comprising: a substrate; and   an interference layer means, disposed on said substrate, for destructively interfering light traversing said layer means towards said substrate, with light traversing said layer means reflected away from said substrate;   wherein said layer means is of a material having a well-defined plasma oscillation; and   wherein said filter further comprised means for directing said light at said filter at an angle between the critical angle of said material for said frequency, and the critical angle of said material for at least one preselected harmonic of said frequency.   
     
     
       11. The filter of claim 10, wherein said filter comprises means for rotatably varying said angle at which said means for directing directs said light at said filter, effective to permit selectable variation of which harmonic of said frequency is said at least one preselected harmonic. 
     
     
       12. The filter of claim 10, wherein said frequency is in the VUV. 
     
     
       13. A method of filtering harmonics of a selected frequency in an optical source, comprising: employing an interference layer on a reflective substrate, said interference layer having a well-defined plasma oscillation; and   directing said light at said interference layer at an angle between the critical angle of said interface layer for said frequency, and the critical angle of said interference layer for at least one selected harmonic of said frequency, effective to cause said interference layer to be highly reflective of said frequency, and highly transmissive of said harmonic.   
     
     
       14. The method of claim 13, wherein said method further comprises selectably rotating said interference layer to cause said directing of said light is at an angle between said critical angle of said interference layer for said frequency, and normal incidence, effective to permit selection of said at least one selected harmonic. 
     
     
       15. The method of claim 13, wherein said frequency is in the VUV.